Method for drying molding containing oxidized metal, method for reducing oxidized metal and rotary hearty type metal reduction furnace

ABSTRACT

The present invention provides: a method for drying compacts containing water so as not to cause explosion and powdering; a method for reducing the compacts after being dried with great efficiency in a rotary-hearth-type reducing furnace; and a rotary-hearth-type metal reducing furnace. In the present invention, when compacts comprising powder containing metal oxide and carbon are dried, the critical value of a water evaporation rate, beyond which explosion occurs, is determined from the size and porosity of the compacts, then the water evaporation rate is controlled to a value not exceeding the critical value and, by so doing, the increase in the internal pressure of the compacts caused by the generation of water vapor is prevented. By the method, the explosion and cracking of the compacts are prevented. Further, when compacts are dried in a rotary-hearth-type reducing furnace, explosion is prevented by controlling the heat supply rate to the compacts through the above method and successively the compacts are incinerated and reduced in the same furnace.

TECHNICAL FIELD

[0001] The present invention relates to: a method for drying compactsproduced by forming powder containing metal oxide and carbon, thesebeing contained in dust, sludge and the like generated, for example, inthe refining and processing of metals; a method for reducing the driedcompacts in a reducing furnace equipped with a rotary hearth; a methodfor drying and successively reducing compacts into metal in a reducingfurnace equipped with a rotary hearth; and further a metal reducingfurnace of a rotary-hearth-type.

BACKGROUND ART

[0002] There are various types of processes as processes for producingreduced iron and ferroalloy and, among those processes,rotary-hearth-type processes are practically used as the processeshaving high productivity. A rotary-hearth-type process is the one mainlycomposed of an incinerator of the type wherein a refractory hearthhaving the toroidal shape of a disc (the center of which is lacked)rotates at a constant speed (hereunder referred to as “rotary furnace”)on rails under fixed refractory ceiling and sidewalls and is used forreducing metal oxide. The diameter of the hearth of a rotary furnace is10 to 50 m and the width thereof is 2 to 6 m.

[0003] Powder containing metal oxide as raw material is mixed with acarbon-type reducing agent, is thereafter formed into raw materialpellets, and is fed to a rotary furnace. An advantage of this is thatthe raw material pellets are laid close and still on the hearth andtherefore the raw material pellets hardly collapse in the furnace.Another advantage thereof is that the problem of powdered raw materialsticking to refractories is avoided and thus the bulk yield of theproduct is high. Further, this method has so far been employedincreasingly since it has a high productivity and allows a lessexpensive carbon-type reducing agent and powdered raw material to beused.

[0004] In addition, a rotary hearth process is also effective forreducing steelmaking dust generated from a blast furnace, a converterand an electric arc furnace and thickener sludge generated from arolling mill and removing impurities, thus is used also as a dustprocessing means, and is a process effective for the recycling ofresources.

[0005] The operation of a rotary hearth process is outlined hereunder.

[0006] Firstly, a carbon-type reducing agent is mixed with metallicoxide comprising raw material of ore, dust and sludge in the amountrequired for reducing the metallic oxide and thereafter the mixture isformed into pellets several to over ten mm in size with a granulatorsuch as a pan-type pelletizer while water is sprayed so that watercontent is about 10%. When the grain sizes of raw material ore and areducing agent are large, they are crushed with a crusher such as a ballmill and thereafter kneaded and granulated.

[0007] The pellets are fed to a rotary hearth and laid in layers. Thepellets laid on the hearth in layers are heated rapidly and incineratedfor 5 to 20 min. at a high temperature of about 1,300° C. During theprocess, metal oxide is reduced by the reducing agent. mixed in thepellets and metal is formed. The metallization ratio after the reductionvaries in accordance with metal to be reduced, and is 95% or more in thecase of iron, nickel, or manganese and 50% or more even in the case ofchromium that is hard to reduce. Further, when dust coming from thesteelmaking industry is processed, impurities such as zinc, lead, alkalimetal, chlorine, etc. are volatilized and removed in accordance with thereducing reaction and therefore the dust may easily be recycled to ablast furnace or an electric arc furnace.

[0008] In such a method of reducing metal and steelmaking dust with arotary hearth, to form raw material and a reducing agent into pellets isan essential requirement. Therefore, it is important to put the mixtureof metal oxide powder as raw material and a reducing agent in the stateliable to be granulated in a preliminary treatment of the raw materialand, for that purpose, various methods such as preliminary crushing ofraw material, kneading in a ball mill and the like are employed.

[0009] As explained above, the reduction of metal oxide by theconventional rotary hearth method is excellent in productivity andproduction costs and is a method for producing metal economically.However, the problems of the method have been that: it is important topelletize raw material and a reducing agent; thus it is necessary eitherto select raw material having an excellent granulation property or toimprove a granulation property by installing an expensive crusher andcrushing raw material. This incurs a large cost.

[0010] Actually, when ore such as iron ore is used as raw material, asthe size of the raw material ore is large, the raw material hasgenerally been crushed into several tens of microns in average diameterand thereafter granulated and produced into pellets. For that reason,the drawbacks of the method have been that: the cost of equipment for acrushing process is high; electricity is required for the operation of acrusher; and the maintenance accompanying the wear of the crusher isexpensive.

[0011] There are some cases where pulverized raw material is used forsaving a crushing cost. However, in such a case, the selection of rawmaterial is limited and thus it has not been a commonly adaptablemethod. For solving the problem, it is effective to use powdered oreafter wet separation, thickener dust from a blast furnace or aconverter, sludge in a scale pit of a rolling process, precipitatedsludge from a pickling process, or the like. However, in those casestoo, a problem has been that a water content is sometimes excessive andtherefore raw material is hardly granulated. In particular, such rawmaterial is composed of very fine powder one to several tens microns insize and, as a result, it is likely to be slurry in the state ofcontaining water, or, even after the raw material is dehydrated with avacuum dehydrator or a filter press, water remains by 30 to 50% and, asit is, it is difficult to granulate because too much water is contained.

[0012] One of the methods for solving the problems is to granulatepowder raw material after it has been dried completely with a heatsource such as a hot blast. However, in this method, the powder rawmaterial forms a pseudo-agglomeration during the drying and thus it isimpossible to granulate it as it is. Therefore, the pseudo-agglomerationof the powder raw material has been crushed, formed in the state of finepowder again, thereafter mixed with other material and water, thengranulated and, thereafter, reduced on a rotary hearth.

[0013] As a result, when the above method is employed, good compacts canbe produced and, if the compacts are dried efficiently, metal oxide isreduced stably. However, by a conventional technology, a method fordrying such compacts, in consideration of the physical state of thecompacts, is not established sufficiently and it has merely beenconsidered that only the drying of the compacts is enough. As a result,there have been problems in that the compacts crack and powdering occursabundantly from the surfaces thereof. Furthermore, when dryingconditions are worse, the compacts have sometimes exploded. Therefore, ameans for solving the problems has been desired for a long time. Here,though a method of drying compacts beforehand is an effective means, theproblem of the method is still that the method requires a heat sourceand a device for exclusively vaporizing the water content, even afterthe compacts are dried, by consuming a large amount of heat and issomewhat disadvantageous economically.

[0014] In particular, when dust or sludge generated in the metalrefining industry and processing industry such as the steelmakingindustry is collected from a wet dust collector or a settling tank, thedust or sludge contains a large amount of water, 90% at the most, andwhen it is attempted to reduce it by a rotary hearth process, the dryingprocess and the succeeding crushing process have been problems.

[0015] As a method of using raw material without granulation in a rotaryhearth process for solving the problems, for example, JapaneseUnexamined Patent Publication No. H11-12619 discloses the method whereinraw material is formed into a tile shape with a compression moldingmachine and used in a rotary hearth process. Even in this methodhowever, there have been problems in using raw material containing alarge amount of water. That is, the problems have been that, as shown inJapanese Unexamined Patent Publication No. H11-12624: the water contentin raw material is required to be adjusted to 6 to 18% and for thatpurpose a drying process is required in addition to a preliminarydehydration process; for that reason the complicated control of a watercontent is required. Further, another problem has been that, in order tocharge such raw material, a complicated charging machine is required asshown in Japanese Unexamined Patent Publication No. H11-12621 and themaintenance cost of the equipment is high.

[0016] Further, when such a type of raw material, containing water, isdirectly charged into a high temperature rotary furnace, the problemshave been that: explosion occurs due to a high water content inaccordance with the evaporation of water; the raw material is pulverizedand taken away with an exhaust gas; and thus the product yielddeteriorates extremely. In a rotary hearth process, the temperature inthe furnace is generally the lowest in the vicinity of a raw materialinlet and about 1,150° C. to 1,200° C. even there. At such a hightemperature, compacts in a wet state entail the problem of explosionaccompanying sudden water evaporation. Even when an explosion does notarise, exfoliation occurs at the corner portions and the surface due tothe eruption of water vapor. Therefore, even though reduction operationcan be carried out, there have been the problems in that the bulk ratioof the reduced product decreases and the ratio of powder generated fromthe compacts increases. As a result, there remain the problems in thatthe ratio of powder metal that is lost in an exhaust gas increasesrelatively and the yield deteriorates.

[0017] The object of the present invention is to provide: a method fordrying efficiently compacts comprising powder raw material containingwater without the generation of explosion or cracking; a method forreducing compacts that makes it possible to reduce the compacts at ahigh yield without the generation of an explosion or the like even whencompacts, in the state of powder containing water, are supplied directlyto a rotary furnace and reduced; and a rotary-hearth-type metal reducingfurnace therefor, those having not so far been realized by conventionalmethods.

DISCLOSURE OF THE INVENTION

[0018] The present invention has been established in view of the aboveproblems and the gist thereof is as follows:

[0019] (1) A method for drying compacts characterized by, in the eventof drying compacts containing powder of metal oxide and carbon and alsowater in mass percentage by not less than 0.2 times the porosity inpercentage, controlling the evaporation rate of water contained in saidcompacts to not more than the value V defined below;

V=300P ² /D,

[0020] where, V means a critical evaporation rate of water (anevaporation rate of water per one dry mass kilogram of compacts(g/kg/sec.)), D a cube root of the volume of a compact (mm), and P aporosity (−).

[0021] (2) A method for drying compacts characterized by, in the eventof drying compacts containing powder of metal oxide and carbon and alsowater in mass percentage by not less than 0.2 times the porosity inpercentage, controlling the rate of heat supply to said compacts to notmore than the value Hin defined below;

Hin=820P ² /D,

[0022] where, Hin means a critical heat supply rate (a heat supply rateper one dry mass kilogram of compacts (kw/kg)), D a cube root of thevolume of a compact (mm), and P a porosity (−).

[0023] (3) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 0.7 g/sec. per one dry mass kilogram of compacts.

[0024] (4) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than1.9 kw per one dry mass kilogram of compacts.

[0025] (5) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 32 to 40% from thestate wherein 6.4 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 1.3 g/sec. per one dry mass kilogram of compacts.

[0026] (6) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 32 to 40% from thestate wherein 6.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than3.5 kw per one dry mass kilogram of compacts.

[0027] (7) A method for drying compacts according to the item (1),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of more than 40 to 55% from thestate wherein 8 mass % or more of water is contained in said compacts,controlling the evaporation rate of water in said compacts to not morethan 2.3 g/sec. per one dry mass kilogram of compacts.

[0028] (8) A method for drying compacts according to the item (2),characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity being more than 40 to 55% fromthe state wherein 8 mass % or more of water is contained in saidcompacts, controlling the rate of heat supply to said compacts to notmore than 6.2 kw per one dry mass kilogram of compacts.

[0029] (9) A method for drying compacts according to any one of theitems (1) to (8), characterized by using powder containing metallicoxide derived from a metal producing process and carbon individually orin mixture as said powder of metal oxide and carbon.

[0030] (10) A method for reducing metal oxide characterized byincinerating and reducing compacts dried by a method according to anyone of the items (1) to (8) at 1,100° C. or higher in arotary-hearth-type furnace wherein compacts containing powder of metaloxide and carbon are loaded on the upper surface of a rotating toroidalhearth and incinerated and reduced by gas combustion heat at the upperinner space of the furnace.

[0031] (11) A method for reducing metal oxide characterized byincinerating and reducing compacts dried by a method according to theitem (9) at 1,100° C. or higher in a rotary-hearth-type furnace whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace.

[0032] (12) A method for reducing metal oxide characterized by, afterdrying compacts by a method according to any one of the items (1) to (8)in a rotary-hearth-type furnace wherein compacts containing powder ofmetal oxide and carbon are loaded on the upper surface of a rotatingtoroidal hearth and incinerated and reduced by gas combustion heat atthe upper inner space of the furnace, successively incinerating andreducing said compacts at 1,100° C. or higher in said furnace.

[0033] (13) A method for reducing metal oxide characterized by, afterdrying compacts by a method according to the item (9) in arotary-hearth-type furnace wherein compacts containing powder of metaloxide and carbon are loaded on the upper surface of a rotating toroidalhearth and incinerated and reduced by gas combustion heat at the upperinner space of the furnace, successively incinerating and reducing saidcompacts at 1,100° C. or higher in said furnace.

[0034] (14) A rotary-hearth-type metal reducing furnace, whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace and thereduced compacts are discharged, characterized in that the area of therotary furnace from the portion where the raw material powder compactsare supplied to a portion 30 to 130 degrees apart from said portion inthe rotation direction is used as the drying zone of the compacts.

[0035] (15) A rotary-hearth-type metal reducing furnace according to theitem (14), characterized in that: an exhaust gas flue is installed at aportion 30 to 130 degrees apart from the portion where the raw materialcompacts are supplied in the rotation direction; and the area from theportion where the raw material compacts are supplied to the portionwhere the exhaust gas flue is installed is used as the drying zone.

[0036] (16) A rotary-hearth-type metal reducing furnace according to theitem (14), characterized in that: a partition having a gap between thelower end thereof and the rotary hearth is installed at a portion 30 to130 degrees apart from the portion where the raw material compacts aresupplied in the rotation direction; and the area from the portion wherethe raw material compacts are supplied to the portion where thepartition is installed is used as the drying zone.

[0037] (17) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped with ameans for cooling the hearth between the portion where the reducedcompacts are discharged and the portion where the raw material compactsare supplied.

[0038] (18) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped withwater cooling means on the ceiling and parts of the sidewalls betweenthe portion where the reduced compacts are discharged and the dryingzone in the furnace.

[0039] (19) A rotary-hearth-type oxidizing metal reducing furnaceaccording to any one of the items (14) to (16), characterized by beingequipped with heating burners on the sidewalls of said drying zone.

[0040] (20) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped with: ameans for cooling the hearth between the portion where the reducedcompacts are discharged and the portion where the raw material compactsare supplied; and water cooling means on the ceiling and parts of thesidewalls between the portion where the reduced compacts are dischargedand the drying zone in the furnace.

[0041] (21) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped with: ameans for cooling the hearth between the portion where the reducedcompacts are discharged and the portion where the raw material compactsare supplied; and heating burners on the sidewalls of the drying zone.

[0042] (22) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped with:water cooling means on the ceiling and parts of the sidewalls betweenthe portion where the reduced compacts are discharged and the dryingzone in the furnace; and heating burners on the sidewalls of the dryingzone.

[0043] (23) A rotary-hearth-type metal reducing furnace according to anyone of the items (14) to (16), characterized by being equipped with: ameans for cooling the hearth between the portion where the reducedcompacts are discharged and the portion where the raw material compactsare supplied; water cooling means on the ceiling and parts of thesidewalls between the portion where the reduced compacts are dischargedand the drying zone in the furnace; and further heating burners on thesidewalls of said drying zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a view showing an example of equipment for reducingmetal oxide, comprising a forming machine, a compact dryer and arotary-hearth-type reducing furnace, to which the present invention isapplied.

[0045]FIG. 2 is a graph showing the relationship between the maximumwater evaporation rate (the critical evaporation rate) in the situationwhere compacts do not explode or the rate of powdering is 10% or lessduring the drying of the compacts and the quotient obtained by dividingthe square of a porosity (P²) by the representative diameter D of acompact. Note that, in the figure, the unit of a critical evaporationrate is g/kg/sec. and the unit of the quotient obtained by dividing thesquare of a porosity (P²) by the representative diameter D of a compactis 1/mm.

[0046]FIG. 3 is a view showing the structure of a rotary-hearth-typemetal reducing furnace having the function of drying compacts in thefurnace and a means for cooling the hearth and the atmosphere in thefurnace according to the present invention.

[0047]FIG. 4 is a view showing the structure of a rotary-hearth-typemetal reducing furnace having the function of drying compacts in thefurnace according to the present invention.

[0048]FIG. 5 is a view showing the outline of an example of equipmentfor reducing metal, comprising an extrusion forming machine and arotary-hearth-type reducing furnace, to which the present invention isapplied.

BEST MODE FOR CARRYING OUT THE INVENTION

[0049]FIG. 1 shows an example of the entire configuration of reducingequipment comprising a rotary-hearth-type reducing furnace and apreliminary treatment apparatus of metallic oxide raw material to be fedto the reducing furnace on the basis of the present invention. Thefigure shows the entire configuration of equipment that producescompacts from powder with a forming machine and reduces them in arotary-hearth-type reducing furnace. The equipment mainly comprises aforming machine 1, a compact dryer 2 and a rotary furnace 3. As aforming machine 1, any type may be employed and a pan-type pelletizer, abriquette forming machine or an extrusion forming machine is generallyused, as described later. Further, a raw material storing apparatus anda product treatment apparatus are included in the equipmentconfiguration, but they are not shown here because they are notimportant in the explanation on the method and equipment of the presentinvention. A powder containing metal oxide and carbon in the state ofcontaining water is formed into compacts with a forming machine 1 andthe formed compacts are dehydrated and dried with a compact dryer 2.Further, the dried compacts are incinerated and reduced in a rotaryfurnace 3. Note that, when compacts can withstand rapid drying or theheat load on the compacts can be reduced in the vicinity of the portionwhere the compacts are loaded in a rotary furnace 3 by theafter-mentioned method of the present invention, the compact dryer 2 maybe omitted.

[0050] The present invention is a method of properly drying compactsproduced in the state of containing water, namely wet compacts, usingpowder containing metal oxide and carbon as raw material. In actualoperation, a method wherein wet compacts are dried by hot blast or thelike with an exclusive dryer or a method wherein wet compacts are driedat a relatively low temperature portion in a rotary furnace where a gastemperature is properly controlled is employed. In order to accomplishthe above object, the present inventors searched for conditions forproperly drying compacts comprising powder containing metal oxide andcarbon. For that purpose, the present inventors carried out theoreticalanalyses on the flow of water vapor in compacts and experiments using asmall hot blast dryer and a box type electric arc furnace.

[0051] Firstly, prior to the experiments, the present inventors carriedout hydrodynamic technological analyses with respect to physicalphenomena of gas flow at the time of evaporation of water in compactsfrom the viewpoint of the analysis of gas flow passing through a narrowpath. Secondly, the present inventors carried out the experiments ofdrying actual compacts and established the treatment standard for dryingcompacts.

[0052] Firstly, on the basis of a physical model showing therelationship between a flow rate and resistance of a fluid flowing infine pores, the present inventors analyzed the pressure at the time whenwater vapor flowed among particles in compacts. From the model analysis,it has been found that a flow resistance per unit length when watervapor flows in pores is inversely proportional to the diameter of thepaths of the pores and is proportional to the flow rate of the watervapor. Further, as a result of the observation of the compacts, it hasbeen found that the path diameter of pores is almost proportional to theporosity. Furthermore, from geometric conditions of the interior ofcompacts, it has been found that the flow rate of water vapor in pathsis proportional to a water vapor generation rate per unit volume ofcompacts and is inversely proportional to the porosity. Here, a porosityis defined as the ratio of the volume of voids to the volume of compactsin the present invention and is generally a value obtained by dividingan apparent specific gravity of compacts by a true specific gravity ofpowder.

[0053] On the basis of the studies in consideration of hydrodynamicconditions and geometric conditions of compacts, when the porosity incompacts is constant, the following relational expression isestablished;

[0054] (Center portion pressure)=A (Path diameter)⁻¹ (Path length)(Watervapor flow rate in path), where, a path length is proportional to thediameter of compacts. Further, the expression is converted into thefollowing expression from aforementioned relation;

[0055] (Center portion pressure)=B (Porosity)⁻¹ (Representativediameter){(Water vapor generation rate per mass)/(Porosity)}=B (Watervapor generation rate per mass)(Porosity)⁻² (Representative diameter).Furthermore, the expression is converted into the following expression;

[0056] (Water vapor generation rate per unit mass)=C (Center portionpressure)(Porosity)²/(Representative diameter), where A, B and C areconstants influenced by the physical state of compacts and the physicalproperties of a gas.

[0057] An internal pressure increases as water vapor is generated. Whenan internal pressure exceeds the pressure under which compacts canwithstand, there arise the problems of explosion, cracking and powderingof the surface of the compacts. The present inventors judged dryingconditions as acceptable when neither explosion nor cracking occurred orthe ratio of powder generated from a surface was 10% or less under thedrying conditions. Here, a powdering ratio is defined by the ratio ofthe mass (mass %) of compacts less than 5 mm in size obtained whencompacts after dried are sieved with a 5 mm sieve to the mass of thecompacts before the sieving. From the results and the above expressions,a water vapor generation rate at an explosion limit was evaluatedquantitatively. The limit (critical pressure) that compacts canwithstand is a value related to the bonding strength of particles in thecompacts and the main factor of the bonding strength is the phenomenonaccompanying the physical adhesiveness among particles. The presentinventors clarified that the bonding strength of particles in compactswas almost constant when a specific binder was not used. Here, a watervapor generation rate (critical evaporation rate) at the time when acenter portion pressure reaches the critical pressure is expressed bythe following expression (a) by formulating an evaluation expression inconsideration of the aforementioned analysis results and the observationresults of the compacts and summarizing the items to be evaluated withconstant values;

V=KP ² /D   (a).

[0058] Further, since an evaporation rate of water is proportional to aheat supply rate, the rate of heat supplied to compacts at a criticalpressure (critical heat supply rate) is shown by the followingexpression (b);

Hin=LP ² /D   (b).

[0059] In the expressions (a) and (b), V means a water vapor evaporationrate per dry mass kg of compacts at the critical pressure (g/kg/sec.),Hin a heat supply rate per dry mass kg of compacts at the criticalpressure (kw/kg), P a porosity (-), and D a cube root of the volume of acompact (mm) that represents the size of the compact. Here, K and L arethe constants. In order to evaluate compacts having different shapes inthe same way, a cube root of the volume of a compact is used for theevaluation of the size of the compact and is hereunder referred to asthe representative diameter.

[0060] In order to determine the constants K and L of the expressions(a) and (b) that stipulated appropriate drying conditions, experimentswere carried out by using a heater for experiments. In the experiments,a hot blast type dryer 5 liters in volume and an electric arc furnace 10liters in volume were used. Raw material for the experiments was powderto be used in a rotary-hearth-type reducing furnace. The powder hadaverage diameters of 4 to 50 microns and contained 63 mass % iron oxideand 15 mass % carbon. The compacts used for the experiments were asfollows; {circle over (1)} spherical compacts produced with a pan-typepelletizer and having a porosity of 22 to 32%, {circle over (2)}compacts produced with a briquette forming machine and having a porosityof over 32 to 40%, and {circle over (3)} columnar compacts produced withan extrusion forming machine and having a porosity of more than 40 to55%. Those forming methods are explained below. The compacts {circleover (1)} were produced by rotating fine powder on a rotating disc, thecompacts {circle over (2)} were formed by using a pair of rolls havingdimples on their surfaces and pushing powder into the dimples while therolls were rotating, and the compacts {circle over (3)} were formed bypushing wet powder into a penetrating nozzle. Here, the representativediameters of the compacts were 5 to 21 mm.

[0061] In the experiments, the heat supply rates to the compacts werechanged variously by changing the hot blast temperature of the hot blasttype dryer or the internal temperature of the box type electric arcfurnace. Among the test results, the cases where explosion did not occurand the powdering loss from the surface was 10% or less (powdering ratiowas 10% or less) were judged to be good drying conditions. The upperlimit of an evaporation rate in the drying under such good dryingconditions was defined as a critical evaporation rate (namely, wateramount evaporated for one second per dry mass kg of compacts) and theheat supply rate at the time was defined as a critical heat supply rate.Thereafter, those values were determined.

[0062] The results are shown in FIG. 2. FIG. 2 shows the relationshipbetween the quotient obtained by dividing the square of a porosity bythe representative diameter of a compact (P²/D) and the criticalevaporation rate (V). The results were subjected to a multipleregression analysis and the value K in the expression (a) was determinedto be 300. Further, the value L in the expression (b) that determined acritical heat supply rate (Hin) was 820. Here, the unit of V wasg/kg/sec., the unit of Hin was kw/kg, and P had no units.

Critical evaporation rate=300P²/D   (1),

Critical heat supply rate=820P²/D   (2).

[0063] Further, in the experiments, when a water ratio was less than 0.2times a porosity, explosion and powdering of a surface did not occureven though the values deviated from the critical values calculated fromthe expressions (1) and (2) because the generated water vapor amount wassmall. Therefore, the present invention is effective in the case where awater ratio is not less than 0.2 times a porosity.

[0064] On the basis of the above analysis results, operations for dryingcompacts are carried out appropriately with actual equipment. Thecompact dryer 2 in the equipment shown in FIG. 1 is a hot blast type andthe heat supply rate to the compacts is controlled. Note that, the dryer2 may be of any type as long as the heat supply rate is controllable.The compacts containing water that have been produced by theaforementioned three methods with the forming machine 1 are supplied tothe compact dryer 2. Here, a heat supply rate not higher than thecritical heat supply rate V obtained from the expression (2) is employedin accordance with the representative diameter and the porosity of thecompacts. It is effective to adjust a heat supply rate by thetemperature and the flow rate of hot blast. The evaporation rate ofwater of the compacts at the time is controlled to a value not higherthan the critical evaporation rate obtained from the expression (1) inaccordance with the representative diameter and the porosity of thecompacts likewise.

[0065] In an actual operation by a rotary hearth process, the compactsused in a rotary furnace 3 have appropriate sizes for improving the heattransfer property in the compacts and maintaining the shapes of thecompacts and a desirable representative diameter is in the range from 5to 21 mm. The reason is as follows: when compacts are too large and inexcess of 21 mm in representative diameter, the inside heat transferbecomes slow, the reduction does not complete within the 7 to 20 min.that is an appropriate reduction time range in a rotary furnace, andcracking occurs at the time of falling; and, on the other hand, when arepresentative diameter is 5 mm or less, the compacts are too small, thecompacts have to be loaded in three to five layers for securing anappropriate amount of the compacts per floor area and, in this case theheat transfer of the compacts in the middle layers is deteriorated, thusthe reducing reaction is also deteriorated.

[0066] The compacts produced with the forming machine 1 are dried withthe compact dryer 2. In the case of dense compacts that are produced bysuch a method as to use a pan-type pelletizer and have a porosity of 22to 32%, the water evaporation rate is controlled to not more than 0.7g/sec. per one dry mass kg of compacts when the compacts having therepresentative diameter of 5 to 21 mm are dried from the state ofcontaining not less than 4.4 mass % water. The water evaporation rate iswithin the critical evaporation rate obtained from the expression (1),thus in the good drying conditions, and does not cause the problems ofexplosion and powdering of the compacts. In this drying method, a heatsupply rate is controlled to not more than 1.9 kw per one dry mass kg ofcompacts. The heat supply rate is relatively low and therefore thecompacts should be dried at a relatively low temperature. A desirabledrying temperature is 400° C. or lower in the case of a hot blast typedryer.

[0067] In the case of compacts that are produced by a method using abriquette forming machine and have a porosity of more than 32 to 40%,the water evaporation rate of the compacts having the representativediameter of 5 to 21 mm is controlled to not more than 1.3 g/sec. per onedry mass kg of compacts when the compacts are dried from the state ofcontaining not less than 6.4 mass % water. In this drying method, theaverage heat supply rate is controlled to not more than 3.5 kJ per onedry mass kg of compacts. In the drying of such compacts, as a somewhathigher heat supply rate is allowed, a desirable drying temperature is inthe range from 200° C. to 550° C. in the case of a hot-blast-type dryer.

[0068] Further, in the case of very porous compacts that are produced bya method using an extrusion forming machine and have a porosity of morethan 40 to 55%, the water evaporation rate of the compacts having therepresentative diameter of 5 to 21 mm is controlled to not more than 2.3g/sec. per one dry mass kg of compacts when the compacts are dried fromthe state of containing not less than 8 mass % water. In the dryingmethod, the average heat supply rate is controlled to not more than 6.3or 6.2 kw per one dry mass kg of compacts. In this drying of thecompacts, as a relatively high heat supply rate is allowable, adesirable drying temperature is in the range from 300° C. to 900° C. inthe case of a hot blast type dryer. Further, if a shorter drying time isdesired, the most appropriate drying temperature is about 800° C.

[0069] In the aforementioned case of drying compacts with a dryer, whenthe compacts are loaded on a rotary furnace 3, the temperature at theportion where the compacts are supplied is high, thus explosion andpowdering caused by rapid heating are problems and, therefore, it isdesirable to control the water content of the compacts after drying tonot more than 1 mass %.

[0070] After the compacts are dried, they are fed to a rotary furnace 3.As the compacts do not contain excessive water, the problems ofexplosion and powdering do not occur even when the heating rate of thecompacts is high in the rotary furnace 3. For example, such a highheating rate that the surface temperature of compacts is raised to1,200° C. for about 3 min. may be acceptable. The compacts areincinerated by being heated in the furnace. As a result, carboncontained in the compacts functions as a reducing agent and reducessolid iron oxide and solid manganese oxide. In this case, the reductionproceeds if the maximum temperature is 1,100° C. or higher, desirably1,200° C. to 1,400° C. and, under this condition, the reducing reactionterminates in 7 to 15 min. The compacts having been incinerated andreduced are discharged from the rotary furnace 3. Thereafter, the hightemperature compacts are cooled with a reduced compact cooling devicenot shown in FIG. 1 and reduced products are obtained. When the reducedproducts are used at a high temperature in an electric arc furnace orthe like, a cooling process may be omitted in some cases.

[0071] There is a method of drying compacts in a rotary furnace 3without the use of a compact dryer 2. An example of the equipmentconfiguration is the one obtained by removing the compact dryer 2 fromthe equipment configuration shown in FIG. 1. An example of the structureof a rotary furnace having such a function is shown in FIG. 3. FIG. 3 isa sectional view of a rotary furnace 3 in the circumferential directionand shows the structure in the vicinity of the drying zone. In thestructure, compacts in a wet state are loaded on the hearth 6 in thedrying zone 5 with the compact feeder 4 and the compacts 12 are driedthere. The hearth 6 rotatively moves toward the right continuously andfeeds the compacts 12 having dried to the reducing zone 7. In thereducing zone 7, the compacts 12 are incinerated and reduced. In themethod of drying compacts 12 in a furnace too, it is necessary toproperly control the heat supply rate at the portion where compacts 12are supplied so that the explosion of the compacts 12 and the powderingof the surface thereof may not occur. In the drying zone 5 too, it isnecessary to control the water evaporation rate of compacts 12 to notmore than the critical evaporation rate (V) and further the heat supplyrate to not more than the critical heat supply rate (Hin).

[0072] In the case of compacts 12 that are produced with a pan-typepelletizer 1 and have the porosity of 22 to 32% and the representativediameter of 5 to 21 mm, the water evaporation rate from the state of notless than 4.4 mass % water is controlled to not more than 0.7 g/sec. perone dry mass kg of compacts 12 at the drying zone 5. Further, the heatsupply rate is controlled to not more than 1.9 or 1.8 kw per one drymass kg of compacts 12. In this method, the compacts 12 should be driedwhile the portion where the compacts 12 are supplied is kept at arelatively low temperature. In order to reduce such dense compacts afterthey are dried, an exclusive dryer, as shown in the equipmentconfiguration of FIG. 1, is generally used for the drying. However, whensuch an exclusive dryer is omitted, the atmospheric temperatures at theportions before and after the portion where the compacts 12 are suppliedin a rotary furnace 3 are lowered and wet compacts 12 are fed. Whencompacts have the porosity of 22 to 32%, a preferable temperature atthis portion is 200° C. to 450° C.

[0073] In a rotary furnace 3, a hearth 6 of a high temperature movescontinuously to the portion where raw material compacts are supplied,namely the raw material compact supply portion and, therefore, theatmospheric temperature usually becomes about 800° C. to 1,000° C. if nomeasures are taken. Therefore, some sort of technological contrivancesare required for lowering the temperature at the raw material compactsupply portion to about 200° C. to 450° C. That is, it is necessary tocool the hearth 6 before the compacts 12 are supplied, avoid introducingexhaust gas generated during the incineration and reduction in thereducing zone 7 to the portion, and compulsorily cool the portionsbefore and after the portion where the compacts 12 are supplied. FIG. 3shows an example of equipment having the structure that absorbs radiantheat of the hearth 6 by installing the water cooling panels 9 on theceiling between the screw discharger 8 for discharging the reducedcompacts 13 and the compact feeder 4 and on the parts of the ceiling inthe drying zone 5. Further, the drying zone 5 and the reducing zone 7are separated from each other with the exhaust gas discharging flue 10so that the high temperature exhaust gas in the reducing zone 7 may notflow in. In this case, at the latter half of the drying zone 5, sinceonly the heat transferred from the hearth 6 is insufficient forsupplying heat to the compacts 12, heating burners 11 may be installedon the sidewalls and heat source for drying may be supplied from theburners.

[0074] In the case of compacts having a porosity of more than 32 to 40%,such as the compacts produced with an aforementioned briquette formingmachine, as long as the heat supply rate is controlled to about 3.5 kwper one dry weight kg of compacts and the water evaporation rate is alsocontrolled up to 1.3 g/sec. per one dry weight kg of compacts, explosionof the compacts 12 and the powdering of the surface thereof do notoccur. The atmospheric temperature of the drying zone 5 in the rotaryfurnace 3 that corresponds to the heat supply rate should be 800° C. orlower. Further, in order to avoid a drying time of 5 min. or longer ofthe compacts 12, a preferable atmospheric temperature is 350° C. orhigher. In this way, the atmospheric temperature of the drying zone 5 islowered for the purpose of lowering the heat supply rate. However, asthe atmospheric temperature may be relatively high, it is not necessaryto compulsorily cool the atmosphere of the drying zone 5 and the hearth6 in many cases. In those cases, an equipment configuration thatexcludes the water cooling panels 9 from the equipment configurationshown in FIG. 3 is employed. Then, the exhaust gas of incineration andreduction is prevented from flowing into the drying zone 5 and also heatis supplemented by combustion using the heating burners 11 in the dryingzone 5. It is desirable to control the heat quantity generated by theheating burners 11 to 0.2 to 0.7 times the heat quantity percircumferential length generated by the burners at the other portions ofthe rotary furnace 3.

[0075] In the case of compacts having a porosity of more than 40 to 55%,such as the compacts produced with an extrusion forming machine or thelike, as long as the heat supply rate is controlled up to 6.2 kw per onedry weight kg of compacts and also the water evaporation rate iscontrolled up to 2.3 g/sec. per one dry weight kg of compacts, explosionand the powdering of a surface do not occur. In the case where such arelatively high heat load is accepted, the temperatures of theatmosphere of the drying zone 5 and the hearth 6 in the rotary furnace 3are controlled to 600° C. to 1,170° C. Here, as the atmospherictemperature is lowered by the influence of water vapor generated fromthe compacts 12 or the like, as long as the temperature range ismaintained, cooling with a specific device is not required. Inversely,strong heating may be required in some cases.

[0076] In order to satisfy the aforementioned conditions and at the sametime to control the atmospheric temperature of the drying zone 5 withhigh accuracy, it is, after all, preferable to employ the equipmentconfiguration shown in FIG. 3, prevent the exhaust gas of incinerationand reduction from flowing into the drying zone 5, and install theheating burners 11 at the portion up to the vicinity of the compactfeeder 4. It is desirable to control the heat quantity generated by theheating burners 11 to 0.5 to 2 times the heat quantity percircumferential length generated by the burners at the other portions ofthe rotary furnace 3. When compacts 12 having a high porosity are driedin a rotary furnace 3 in this way, the heat transfer rate may berelatively high and a device having a simplified structure may beaccepted, and therefore the method is excellent in equipment cost andoperation cost.

[0077] The time period spent for drying compacts 12 in the drying zone 5is controlled to 60 to 300 sec. When drying is finished in the shorttime of 60 sec. or less, the heat amount supplied for drying thecompacts 12 is too much in many cases and the problem arising in thecase of large compacts 12 is that water remains in the core portions ofthe compacts 12. On the other hand, the drying of the compacts 12 iscompleted within 300 sec. in most cases and therefore the dryingexceeding 300 sec. causes a large energy loss and a larger equipmentsize. For those reasons, a preferable drying time is in the range from60 to 300 sec. As stated above, when various compacts 12 havingdifferent porosities are dried, the atmospheric temperature of thedrying zone 5 is controlled to 200° C. to 1,170° C. The atmospherictemperature is changed in accordance with the porosity of the compacts12.

[0078] The length of the drying zone 5 in a rotary furnace 3 is definedby the length from the portion where raw material powder compacts 12 aresupplied to a portion 30 to 130 degrees away from the portion in therotation direction. The reasons are that: the drying time is in therange from 60 to 300 sec. and the reducing time is in the range from 8to 20 min. (480 to 1,200 sec.); it is difficult to control theatmospheric temperature of the drying zone 5 independently when thelength of the drying zone 5 is the length that corresponds to 30 degreesor less in arc angle; and others.

[0079] It is an effective means for the control of the atmospherictemperature of the drying zone 5 to install an exhaust gas dischargingflue 10 at the boundary between the drying zone 5 and the reducing zone7 so that the high temperature exhaust gas generated in the reducingzone 7 may not flow into the drying zone 5 as stated above and shown inFIG. 3. In this case, when the atmospheric temperature of the dryingzone 5 lowers excessively, supplementary combustion is applied with theheating burners 11 on the sidewalls. On the other hand, when theatmospheric temperature of the drying zone 5 is excessively high, thestructure for cooling the hearth 6 and the atmosphere is required asshown in FIG. 3. As a method of cooling the hearth 6, there is themethod wherein the hearth 6 is cooled by installing water cooling panels9 on the ceiling between the screw discharger 8 and the compact feeder 4as stated above. In this case, the temperature of the hearth 6 islowered by absorbing the radiant heat from the hearth 6 that has beenexposed after the reduced compacts 13 have been discharged with metalwater cooling panels 9. By this method, the surface temperature of thewater cooling panels 9 is about 300° C. and the surface temperature ofthe hearth 6 is lowered to about 900° C. or lower with the cooling for30 to 50 sec. Further, a method of spraying water on the hearth 6through spray nozzles or the like at the portion upstream the compactfeeder 4 is also effective for the cooling of the hearth 6.

[0080] In this way, by the method of installing an exhaust gasdischarging flue 10 at the boundary between the drying zone 5 and thereducing zone 7, it becomes possible to prevent a high temperatureexhaust gas from flowing into the drying zone 5, thus lowering theatmospheric temperature of the drying zone 5 effectively, and to controlthe temperature with high accuracy. In contrast, in the case where theatmospheric temperature of the drying zone 5 may be 500° C. or higher ora similar case, there is also the method of installing a partition 14having a gap at the lower portion at the boundary between the dryingzone 5 and the reducing zone 7 as shown in FIG. 4. By the effect of thepartition 14, the drying zone 5 and the reducing zone 7 are separatedinto independent zones and it becomes easy to control the atmospherictemperatures individually. When the control of an atmospherictemperature with high accuracy is not required, a partition 14 may notbe required and a method of controlling the atmospheric temperature ofthe drying zone 5 independently from the reducing zone 7 may beemployed.

[0081] Next, a typical example of equipment configuration in the case ofusing an extraction forming machine, which is the most economical methodas a process of excluding a dryer 2, is shown in FIG. 5. The methods forproducing, drying and reducing compacts with this equipment areexplained hereunder. Firstly, raw material comprising metal oxide powderin the state of containing not less than 50 mass % water and pulverizedreducing agent mainly composed of carbon is mixed and contained in amixing pit 15. As the metal oxide raw material, ore powder includingiron ore powder, manganese ore powder, chromium ore powder, etc., dustfrom a refining furnace and sludge from a rolling process, those beinggenerated in the metal producing industry, and others are used. Inparticular, sludge generated in the metal producing industry is the mostsuitable raw material because it contains about 70% water by nature.

[0082] The solid-liquid mixture of the raw material is stirred and mixedwell in the mixing pit 15. The solid-liquid mixture is transported to adehydrator 17 with a slurry transportation pump 16, dehydrated to thewater content of 15 to 27 mass % with the dehydrator, and formed intowater contained aggregates of the raw material mixture. As thedehydrator 17, a dehydrator of the type wherein solid-liquid mixture ispoured on a filter cloth that moves in a circulatory manner and squeezedwith a pair of press rolls installed above and under the filter cloth, afilter press, a centrifugal dehydrator or the like is preferably used.The produced water contained aggregates is fed to an extrusion formingmachine 18 and formed into compacts while water is contained. It ispreferable that the compacts 12 have a diameter of about 8 to 20 mm anda representative diameter of 5 to 21 mm. The compacts 12 is configuredso that water vapor may be likely to be extracted and thus the compactsmay be hardly exploded in a rotary furnace 3. Specifically, the porosityof the compacts 12 is controlled to 40 to 55%.

[0083] The compacts 12 are fed to the rotary furnace 3 in the state ofcontaining 15 to 27 mass % water. In the rotary furnace 3, the compacts12 are charged on a hearth 6 and thereafter dried in a drying zone 5while the heating rate is controlled. Specifically, the compacts 12 aredried for 60 to 300 sec. at 600° C. to 1,170° C. The compacts 12 fromwhich water has been removed (dried) in the drying zone 5 move togetherwith the hearth 6 in the furnace, are transferred to a reducing zone 7having a high temperature, are subjected to reducing reaction activelyat the time when the temperature of the compacts 12 exceeds 1,100° C.,and most of the metal oxide of the compacts 12 is transformed intometal. The reduced compacts 13 are discharged from the hearth 6 with ascrew discharger 8. The reduced compacts 13 thus produced are used asraw material for a metal reducing furnace or a metal refining furnaceincluding an electric arc furnace or a blast furnace.

EXAMPLES

[0084] The examples of the operation for drying and reducing compactscomprising the powder of metal oxide and carbon according to the presentinvention are explained hereunder. Firstly, Table 1 shows the results ofExamples 1 to 3 obtained by drying compacts in an exclusive dryer 2 andthereafter incinerating and refining them in a rotary furnace 3. Here,the treatment conditions of Examples 1 to 3 are as follows. The rawmaterial powder comprised 63 mass % iron oxide and 15 mass % carbon andthe average diameter of the powder was 11 microns. The compacts wereformed from the powder with three kinds of devices; a pan-typepelletizer, a briquette forming machine and an extraction formingmachine. The compacts produced by those methods were dried in the dryingfurnace 2 by controlling the water evaporation rate to not more than V(critical evaporation rate) and the heat supply rate to not more thanHin (critical heat supply rate). Further, the compacts after dried wereincinerated and reduced in the rotary furnace 3. In any of the reducingtreatments, the reducing time was 15 min. and the atmospherictemperature during the reducing was 1,320° C. In contrast, in the caseof Comparative Examples, the same compacts as Examples 1 to 3 were usedand subjected to incineration and reduction. However, they were dried bycontrolling the water evaporation rate to not less than V (criticalevaporation rate) or the heat supply rate to not less than Hin (criticalheat supply rate). The other conditions were identical to Examples 1 to3. The results are shown in Table 2.

[0085] Here, a powdering ratio was defined by the ratio of the mass(mass %) of the compacts less than 5 mm in size obtained by sieving thecompacts after being dried with a sieve 5 mm in mesh to the total massof the compacts before sieving. Likewise, a bulk yield of reducedproducts was defined by the ratio of the mass (mass %) of the compactsnot less than 5 mm in size obtained by sieving the compacts afterreduced with a sieve 5 mm in mesh to the total mass of the compactsbefore sieving, and an iron metallization ratio was defined by the ratioof the mass (mass %) of the metallic iron in the reduced products to thetotal iron mass. TABLE 1 Example 1 Example 2 Example 3 Forming machinePan-type Briquette Extrusion pelletizer forming forming machine machineCompact Spherical Almond- Columnar shaped Size *) (mm) 15 18 17 Porosity(%)   27%   33%   47% Water content (mass %) 11.5% 14.6% 21.2% Criticalvalue of drying V value (g/kg · sec.) 1.5 1.8 3.9 Hin value (kw/kg) 4.05.0 10.7 Drying result Actual water evaporation 0.77 1.3 2.7 rate (g/kg· sec.) Actual heat supply rate (kw/kg) 2.1 3.7 7.5 Evaluation of dryingstate Existence of explosion None None None Powdering ratio (mass %) 3.9%  2.6%  3.3% Result of reduced product Iron metallization ratio(mass %)   85%   88%   88% Bulk product yield (mass %)   92%   88%   86%

[0086] Example 1 shows the results of the operation in the case of usingcompacts a produced with a pan-type pelletizer and having a porosity of27%, which is relatively dense. The values of V and Hin computed fromthe size and the porosity of the compacts were 1.5 g/kg/sec. and 4.0kw/kg, respectively. Meanwhile, the actual water evaporation rate andheat supply rate were 0.77 g/kg/sec. and 2.1 kw/kg, respectively.Therefore, explosion was not observed since the water evaporation ratewas lower than the critical value and, further, the powdering from thecompact surfaces was as low as 3.9%. As a result of reducing thecompacts, the iron metallization ratio was as high as 85% and the bulkyield of the products was as good as 92%.

[0087] In Example 2, compacts produced with a briquette forming machineand having a porosity of 33% were used. The values of V and Hin computedfrom the size and the porosity of the compacts were 1.8 g/kg/sec. and5.0 kw/kg, respectively. Meanwhile, the actual water evaporation rateand heat supply rate were as low as 1.3 g/kg/sec. and 3.7 kw/kg,respectively. Therefore, explosion was not observed and further thepowdering from the compact surfaces was as low as 2.6%. As a result ofreducing the compacts, the iron metallization ratio was as high as 88%and the bulk yield of the products was as good as 88%.

[0088] In Example 3, compacts produced with an extrusion forming machineand having a porosity of 47%, which is a low packing density, were used.The values of V and Hin computed from the size and the porosity of thecompacts were 3.9 g/kg/sec. and 10.7 kw/kg, respectively. Meanwhile, theactual water evaporation rate and heat supply rate were as low as 2.7g/kg/sec. and 7.5 kw/kg, respectively. Therefore, explosion was notobserved and further the powdering from the compact surfaces was as lowas 3.3%. As a result of reducing the compacts, the iron metallizationratio was as high as 88% and the bulk yield of the products was as goodas 86%. As it has been clarified above, as long as the drying conditionsare controlled within the ranges stipulated in the present invention,the compacts are dried well and also reduced properly. TABLE 2Comparative Comparative Comparative Example 1 Example 2 Example 3Forming machine Pan-type Briquette Extrusion pelletizer forming formingmachine machine Compact Spherical Almond- Columnar shaped Size *) (mm)15 18 17 Porosity (%)   27%   33%   47% Water content (mass %) 11.5%14.6% 21.2% Critical value of drying V value (g/kg · sec.) 1.5 1.8 3.9Hin value (kw/kg) 4.0 5.0 10.7 Drying result Actual water evaporation2.5 2.2 5.5 rate (g/kg · sec.) Actual heat supply rate 5.8 5.9 13.9(kw/kg) Evaluation of drying state Existence of explosion OccurredOccurred None Powdering ratio (mass %)   88%   76%   37% Result ofreduced product Iron metallization ratio Not Not 56% (mass %) operableoperable Bulk product yield (mass %) — — 53%

[0089] On the other hand, in Comparative Examples 1 to 3, the resultswere obtained by drying the same compacts as Examples 1 to 3 under theconditions deviating from those stipulated in the present invention andthen reducing 10 them. In any of Comparative Examples, as the waterevaporation rate and the heat supply rate of the compacts were largerthan the critical values respectively, the compacts were not driedproperly. In the cases of Comparative Examples 1 and 2, the compactsexploded and 76 to 88% of the compacts were decomposed into powder. As aresult, the reduction operation was not properly carried out in therotary furnace 3. Further, in Comparative Example 3, the results wereobtained by drying compacts produced with an extrusion forming machineand having a high porosity and then reducing them. In the drying of thecompacts too, the water evaporation rate and the heat supply rate of thecompacts were larger than the critical values shown by V and Hinrespectively. As a result, though explosion did not occur, 37% of thecompacts were decomposed into powder. As a result of incinerating andreducing the mixture of the bulk and powder of the compacts in therotary furnace 3, the powdered portions were influenced by thereoxidation caused by carbon dioxide gas in the atmosphere, the ironmetallization ratio was low, and the bulk yield of the products was alsolow.

[0090] Next, Table 3 shows the results of Examples 4 to 6 that are theexamples of the operations carried out by the method wherein thecompacts are dried in the rotary furnace 3 as shown in FIG. 3 or 4. Thetreatment conditions of Examples 1 to 3 are as follows. The raw materialpowder comprised 63 mass % iron oxide and 15 mass % carbon and theaverage diameter of the powder was 11 microns, which were the same asExamples 1 to 3. The compacts were formed likewise from the powder withthree kinds of devices; a pan-type pelletizer, a briquette formingmachine and an extraction forming machine. In the drying of the compactsin the furnace, the water evaporation rate was controlled to not morethan V (critical evaporation rate) and the heat supply rate to not morethan Hin (critical heat supply rate). Further, the compacts after beingdried were successively incinerated and reduced in the same furnace. Thereducing time was 13 min. and the atmospheric temperature during thereduction was 1,300° C. TABLE 3 Example 4 Example 5 Example 6 Formingmachine Pan-type Briquette Extrusion pelletizer forming forming machinemachine Compact Spherical Almond- Columnar shaped Size *) (mm) 17 20 20Porosity (vol. %)   27%   33%   47% Water content (mass %) 11.5% 14.6%21.2% Drying zone of rotary furnace Structure of drying zone ReducingReducing Reducing zone and zone and zone and drying zone drying zonedrying zone are are are separated by separated separated exhaust gas byexhaust by exhaust discharging gas gas flue, discharging dischargingceiling is flue, flue, equipped heating heating with cooling burners areburners are means. installed installed Drying zone resident 200 160 100time (sec.) Drying zone temperature 250-450° C. 450-750° C. 700-950° C.Critical value of drying V value (g/kg · sec.) 1.3 1.6 3.3 Hin value(kw/kg) 3.5 4.5 9.1 Drying result Actual water evaporation 0.67 1.1 2.6rate (g/kg · sec.) Actual heat supply rate 1.8 3.1 7.5 (kw/kg)Evaluation of drying state Existence of explosion None None NonePowdering ratio (mass %)  5.1%  5.9%  3.1% Result of reduced productIron metallization ratio   84%   81%   83% (mass %) Bulk product yield  94%   91%   95% (mass %)

[0091] Example 4 is an example of the operation in the case of usingspherical compacts produced with a pan-type pelletizer and having aporosity of 27%, which is relatively dense. The compacts have a lowporosity and they explode easily when a water evaporation rateincreases. Therefore, the atmospheric temperature in the drying zone 5was controlled in the range from the lowest temperature of 250° C. tothe highest temperature of 450° C. For that purpose, the exhaust gasdischarging flue 10 was installed between the drying zone 5 and thereducing zone 7 so that exhaust gas generated in the reducing zone 7might not flow into the drying zone 5. Further, in order to lower theatmospheric temperature and the hearth 6, the ceiling between the screwdischarger 8 and the compact feeder 4 at the compact supply portion andparts of the ceiling in the drying zone 5 were equipped with watercooling means. As a result, the heat supply rate to the compacts couldbe reduced to 1.8 kw/kg, lower than Hin, and the water evaporation ratecould also be reduced to 0.67 g/kg/sec., lower than V. The reducingtreatment was also good and the powdering ratio was as low as 5.1% andthe iron metallization ratio and the bulk product yield were high.

[0092] Example 5 is an example of the operation in the case of usingalmond-shaped compacts produced with a briquette forming machine andhaving a porosity of 33%. In this case, the heat supply rate to thecompacts was controlled to not more than Hin and the water evaporationrate was also controlled to not more than V so that the compacts mightnot cause the problems of the explosion or powdering. For this purpose,the exhaust gas discharging flue 10 was installed between the dryingzone 5 and the reducing zone 7 so that exhaust gas generated in thereducing zone 7 might not flow into the drying zone 5. Here, in the caseof these compacts, the amount of the water vapor generated from thecompacts is relatively large and, therefore, the atmospheric temperaturein the drying zone 5 lowers in excess of the target temperature in somecases. For this reason, heat was supplemented with the heating burners11 installed on the sidewalls and the atmospheric temperature wascontrolled in the range from the lowest temperature of 450° C. to thehighest temperature of 750° C. As a result, the water evaporation ratewas 1.1 g/kg/sec., lower than V. The iron metallization ratio and thebulk yield of the reduced products were good.

[0093] Example 6 is an example of the operation in the case of usingcolumnar compacts produced with an extrusion forming machine and havinga porosity of 47%. In this case too, the heat supply rate to thecompacts was controlled to not more than Hin and the water evaporationrate was also controlled to not more than V so that the compacts mightnot cause the problems of the explosion or powdering. For this purpose,the exhaust gas discharging flue 10 was installed between the dryingzone 5 and the reducing zone 7, similarly to Example 5. The compacts ofExample 6 contained a large amount of water and therefore theatmospheric temperature in the drying zone 5 dropped considerably due tothe water vapor. To cope with the problem, heat was supplemented withthe heating burners 11 installed on the sidewalls and the atmospherictemperature was controlled in the range from the lowest temperature of700° C. to the highest temperature of 950° C. As a result, the waterevaporation rate was 3.3 g/kg/sec., lower than V. In this operation too,the iron metallization ratio and the bulk yield of the reduced productswere good.

[0094] Next, as Example 7, iron oxide and sludge containing carbon in aquantity generated in the processes of the steelmaking industry wereused as the raw material and formed into compacts and the compacts werereduced by using the reducing equipment shown in FIG. 5. The rawmaterial used in the operation had the average diameter of 9 microns andthe water content of 21%. The porosity of the compacts produced with theextraction forming machine was 44% and the representative diameterthereof was 15 mm. In Example 7, the temperature of the drying zone 5was controlled in the range from 890° C. to 1,020° C. and the length ofthe drying zone 5 was 150 sec. in terms of the transit time of thehearth 6. As a result of drying the compacts under those conditions, theproblems of the explosion and powdering of the compacts did not occur.The highest temperature in the reducing zone 7 was 1,300° C. and thereducing time was 13 min. The bulk yield of the reduced product ofExample 7 was as high as 91% and the iron loss to dust was as low as1.7%. Further, the iron metallization ratio was 88% and thus thereduction was also good.

INDUSTRIAL APPLICABILITY

[0095] The present invention makes it possible to dry compacts, made ofpowder containing water, properly and to reduce metal oxide economicallyin a rotary hearth reduction process. Further, the present invention iseffective for the processing of sludge comprising metal oxide containingwater in quantity and dust containing carbon.

1. A method for drying compacts characterized by, in the event of dryingcompacts containing powder of metal oxide and carbon and also water inmass percentage by not less than 0.2 times the porosity in percentage,controlling the evaporation rate of water contained in said compacts tonot more than the value V defined below; V=300P ² /D, where, V means acritical evaporation rate of water (an evaporation rate of water per onedry mass kilogram of compacts (g/kg/sec.)), D a cube root of the volumeof a compact (mm), and P a porosity (−).
 2. A method for drying compactscharacterized by, in the event of drying compacts containing powder ofmetal oxide and carbon and also water in mass percentage by not lessthan 0.2 times the porosity in percentage, controlling the rate of heatsupply to said compacts to not more than the value Hin defined below;Hin=820P ² /D, where, Hin means a critical heat supply rate (a heatsupply rate per one dry mass kilogram of compacts (kw/kg)), D a cuberoot of the volume of a compact (mm), and P a porosity (−).
 3. A methodfor drying compacts according to claim 1, characterized by, in the eventof drying compacts having a cube root of the volume of 5 to 21 mm and aporosity of 22 to 32% from the state wherein 4.4 mass % or more of wateris contained in said compacts, controlling the evaporation rate of waterin said compacts to not more than 0.7 g/sec. per one dry mass kilogramof compacts.
 4. A method for drying compacts according to claim 2,characterized by, in the event of drying compacts having a cube root ofthe volume of 5 to 21 mm and a porosity of 22 to 32% from the statewherein 4.4 mass % or more of water is contained in said compacts,controlling the rate of heat supply to said compacts to not more than1.9 kw per one dry mass kilogram of compacts.
 5. A method for dryingcompacts according to claim 1, characterized by, in the event of dryingcompacts having a cube root of the volume of 5 to 21 mm and a porosityof more than 32 to 40% from the state wherein 6.4 mass % or more ofwater is contained in said compacts, controlling the evaporation rate ofwater in said compacts to not more than 1.3 g/sec. per one dry masskilogram of compacts.
 6. A method for drying compacts according to claim2, characterized by, in the event of drying compacts having a cube rootof the volume of 5 to 21 mm and a porosity of more than 32 to 40% fromthe state wherein 6.4 mass % or more of water is contained in saidcompacts, controlling the rate of heat supply to said compacts to notmore than 3.5 kw per one dry mass kilogram of compacts.
 7. A method fordrying compacts according to claim 1, characterized by, in the event ofdrying compacts having a cube root of the volume of 5 to 21 mm and aporosity of more than 40 to 55% from the state wherein 8 mass % or moreof water is contained in said compacts, controlling the evaporation rateof water in said compacts to not more than 2.3 g/sec. per one dry masskilogram of compacts.
 8. A method for drying compacts according to claim2, characterized by, in the event of drying compacts having a cube rootof the volume of 5 to 21 mm and a porosity of more than 40 to 55% fromthe state wherein 8 mass % or more of water is contained in saidcompacts, controlling the rate of heat supply to said compacts to notmore than 6.2 kw per one dry mass kilogram of compacts.
 9. A method fordrying compacts according to claim 1, characterized by using powdercontaining metallic oxide derived from a metal producing process andcarbon individually or in mixture as said powder of a metal oxide andcarbon.
 10. A method for reducing metal oxide characterized byincinerating and reducing compacts dried by a method according to claim1 at 1,100° C. or higher in a rotary-hearth-type furnace whereincompacts containing powder of metal oxide and carbon are loaded on theupper surface of a rotating toroidal hearth and incinerated and reducedby gas combustion heat at the upper inner space of the furnace.
 11. Amethod for reducing metal oxide characterized by incinerating andreducing compacts dried by a method according to claim 9 at 1,100° C. orhigher in a rotary-hearth-type furnace wherein compacts containingpowder of metal oxide and carbon are loaded on the upper surface of arotating toroidal hearth and incinerated and reduced by gas combustionheat at the upper inner space of the furnace.
 12. A method for reducingmetal oxide characterized by, after drying compacts by a methodaccording to claim 1 in a rotary-hearth-type furnace wherein compactscontaining powder of metal oxide and carbon are loaded on the uppersurface of a rotating toroidal hearth and incinerated and reduced by gascombustion heat at the upper inner space of the furnace, successivelyincinerating and reducing said compacts at 1,100° C. or higher in saidfurnace.
 13. A method for reducing metal oxide characterized by, afterdrying compacts by a method according to claim 9 in a rotary-hearth-typefurnace wherein compacts containing powder of metal oxide and carbon areloaded on the upper surface of a rotating toroidal hearth andincinerated and reduced by gas combustion heat at the upper inner spaceof the furnace, successively incinerating and reducing said compacts at1,100° C. or higher in said furnace.
 14. A rotary-hearth-type metalreducing furnace, wherein compacts containing powder of metal oxide andcarbon are loaded on the upper surface of a rotating toroidal hearth andincinerated and reduced by gas combustion heat at the upper inner spaceof the furnace and the reduced compacts are discharged, characterized inthat the area of the rotary furnace from the portion where the rawmaterial powder compacts are supplied to a portion 30 to 130 degreesapart from said portion in the rotation direction is used as the dryingzone of the compacts.
 15. A rotary-hearth-type metal reducing furnaceaccording to claim 14, characterized in that: an exhaust gas flue isinstalled at a portion 30 to 130 degrees apart from the portion wherethe raw material compacts are supplied in the rotation direction; andthe area from the portion where the raw material compacts are suppliedto the portion where the exhaust gas flue is installed is used as thedrying zone.
 16. A rotary-hearth-type metal reducing furnace accordingto claim 14, characterized in that: a partition having a gap between thelower end thereof and the rotary hearth is installed at a portion 30 to130 degrees apart from the portion where the raw material compacts aresupplied in the rotation direction; and the area from the portion wherethe raw material compacts are supplied to the portion where thepartition is installed is used as the drying zone.
 17. Arotary-hearth-type metal reducing furnace according to claim 14,characterized by being equipped with a means for cooling the hearthbetween the portion where the reduced compacts are discharged and theportion where the raw material compacts are supplied.
 18. Arotary-hearth-type metal reducing furnace according to claim 14,characterized by being equipped with water cooling means on the ceilingand parts of the sidewalls between the portion where the reducedcompacts are discharged and the drying zone in the furnace.
 19. Arotary-hearth-type oxidizing metal reducing furnace according to claim14, characterized by being equipped with heating burners on thesidewalls of said drying zone.
 20. A rotary-hearth-type metal reducingfurnace according to claim 14, characterized by being equipped with: ameans for cooling the hearth between the portion where the reducedcompacts are discharged and the portion where the raw material compactsare supplied; and water cooling means on the ceiling and parts of thesidewalls between the portion where the reduced compacts are dischargedand the drying zone in the furnace.
 21. A rotary-hearth-type metalreducing furnace according to claim 14, characterized by being equippedwith: a means for cooling the hearth between the portion where thereduced compacts are discharged and the portion where the raw materialcompacts are supplied; and heating burners on the sidewalls of thedrying zone.
 22. A rotary-hearth-type metal reducing furnace accordingto claim 14, characterized by being equipped with: water cooling meanson the ceiling and parts of the sidewalls between the portion where thereduced compacts are discharged and the drying zone in the furnace; andheating burners on the sidewalls of the drying zone.
 23. Arotary-hearth-type metal reducing furnace according to claim 14,characterized by being equipped with: a means for cooling the hearthbetween the portion where the reduced compacts are discharged and theportion where the raw material compacts are supplied; water coolingmeans on the ceiling and parts of the sidewalls between the portionwhere the reduced compacts are discharged and the drying zone in thefurnace; and further heating burners on the sidewalls of said dryingzone.